Since they tell you to arrive "at least" 15 minutes prior to your ADR time, I don't think 20 or 25 minutes early is pushing it. Now, if you get there an hour early, that's another story. I have noticed that at some of the busier times they have prioritized guests according to their ADR time, so if you check in 25 minutes early, and the next person in line is 15 minutes early for their ADR, they may be seated before you. I've only seen that a few times, but I have seen it.